1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, and a facsimile machine, and more particularly, to an electrophotographic image forming apparatus.
2. Discussion of the Background
Various types of transfer materials, such as copy sheets, are used in image forming apparatuses such as copiers, printers, facsimile machines, and the like. Depending on the purpose of image formation, a transfer material that has a rough surface, that is, a material having surface asperities, is sometimes preferred. However, when such a rough-surfaced transfer material (hereinafter “rough material”) is used for printing, there is a possibility that an image formed thereon might be disturbed during the transfer process, which is a process of transferring a toner image onto the transfer material. In particular, where the surface asperities are relatively significant, gaps are created between the transfer material and the toner image that is formed on the toner image carrier that carries the toner image. Those gaps can destabilize a transfer electric field used to transfer the toner image from the image carrier, resulting in image failures such as white voids in which toner is partly absent, inconsistencies in lightness, and image density unevenness.
Several approaches described below have been proposed to prevent such image failures.
In one known image forming apparatus, an image carrier such as an intermediate transfer belt is vibrated by ultrasound to weaken adhesion between the toner and the image carrier so that the image can be transferred to the transfer material even if the electric field is unstable in the gap portions. However, in such an image forming apparatus including a vibration member, vibration noise is generated, which can annoy users. Additionally, the vibration tends to shorten the working life of other members such as the image carrier.
In another known image forming apparatus, to print high quality multicolor images, the image carrier such as an intermediate transfer belt includes an elastic layer, and its surface that carries toner is designed to have a surface micro hardness within a predetermined range to follow the asperities in the surface of the recording medium, thus reducing the gaps. However, in this known image forming apparatus, the cost of forming the elastic layer on the image carrier is relatively high. Further, this configuration cannot accommodate tiny gaps.
In another known approach, the image forming apparatus is a direct transfer type. The image forming apparatus includes an information acquisition mechanism that acquires information related to a surface structure of the transfer material, and a control mechanism that varies the degree of toner adhesion depending on the degree of surface roughness of the transfer material as indicated by the information related to the surface structure of the transfer material. Then, when a sheet reading mechanism in the information acquisition mechanism detects that the sheet has a rough surface, the control mechanism increases a transfer bias that is applied to a transfer nip during the transfer process by the control mechanism so as to increase the amount of the toner adhering to the sheet.
However, in this known image forming apparatus, although the sheet reading mechanism judges whether the surface of the sheet is rough or smooth, the judgment is made in accordance with the entire surface of transfer material, and adhesion is adjusted by varying the transfer bias.
Therefore, because this mechanism does not adjust the toner amount in accordance with localized concavities in the surface of the sheet, the overall color reproducibility has a problem, and the solid shaded and halftone areas are not balanced.
Additionally, because the toner amount is increased for the entire transfer material, the developer is consumed in excess, which is inefficient.